I am new to diving. I purchased an underwater writing tablet, and am unsure on how to erase on it. Is there some sort of coating on the tablet that I need to worry about, or will an eraser do the trick?

As long as you are using a pencil it will erase with a regular eraser. I don't think it will work well under water (slate & eraser will need to be dry). I've used soap and a brillo pad to erase as well.